Absolute Calibration of the Jason-1 Altimeter Using UK Tide Gauges

A consistency between seasonal fluctuation of actual sea surface height (SSH) and those caused by mass and density variations in gyre-scale regions is examined. The SSH obtained from satellite altimetry (altimetric SSH) is adopted as the actual SSH. SSH caused by mass variation (mass-related SSH) is simulated using a barotropic global ocean model forced by water flux, wind stress and surface pressure. SSH caused by density variation (steric SSH) is calculated from water density profile, i.e. temperature and salinity profiles. The model SSH well represents mass-related SSH for gyre-scale regional means, and seasonal fluctuation of the altimetric SSH corrected for the model SSH is similar to that of steric SSH above a pressure level larger than 300 dbar. The results indicate that the mass-related SSH does not much respond to the baroclinic adjustment to the seasonally varying wind stress curl. The mass-related SSH forced by wind stress and surface pressure should be accounted for regional evaluation, though it is not necessary for global mean evaluation. Detection of steric SSH from altimetric SSH would be useful for assimilation approaches in which the altimetric SSH is treated as the variable reflecting subsurface temperature and salinity.

Global Positioning System (GPS) information is stored in EXchangeable Image File Format (EXIF) in the image file. It is considered personal information; therefore, sharing it without the owner’s consent could lead to personal information disclosure. GPS information is embedded on the images uploaded by many people on social network services or stored in the Cloud. Images must be encrypted to prevent GPS information from being disclosed. However, there are no existing methods to encrypt them in EXIF. Using block cipher for encryption can exceed the tag value range of EXIF. In this paper, we propose a method to encrypt the GPS information of an image file using Format-Preserving Encryption, which preserves the length and format of the plaintext. We confirmed that GPS information can be secured by encrypting it in image files.

This study presents results of calibration/validation (C/V) of Jason-1 and Jason-2 satellite altimeters over Lake Issykkul located in Kyrgyzstan, which was chosen as a dedicated radar altimetry C/V site in 2004. The objectives here are to estimate absolute altimeter biases and to quantify the altimetry instrument error budgets for lakes studies, including errors associated with the atmosphere media delay corrections of the radar signal, and with the different retracking mode used over “nonocean” surface. The C/V is conducted using various equipments, for example, GPS local network, moving GPS along the satellites tracks over the Lake Issykkul, in situ level gauges and weather stations. The absolute bias obtained for Jason-1 and Jason-2 from field campaigns conducted in 2008, 2009 and 2010 are 96 ± 40 mm and 162 ± 42 mm, respectively. The bias calculated for Jason-1 is in close agreement with estimates done at other dedicated C/V sites in the ocean, but the Jason-2 bias still differs by 1–2 cm with ocean C/V estimates. The relative bias (Jason-2 minus Jason-1) deduced from measurements during the tandem mission is estimated at 81 ± 10 mm. The accuracy of Jason-1 and Jason-2 for the determination of the lake level variations as compared to the historical in situ gauges of the Lake Issykkul is 33 and 31 mm RMS, respectively. The bias between the two modes of retracking available on Jason-2 Geophysical Data Records (GDRs) has been estimated over the two first years of Jason-2 data over the Lake Issykkul. We found an average value of 234 ± 40 mm, which is in good agreement with results obtained in a previous study on the Envisat satellite (Crétaux et al. 2009). Lake level studies using Jason-2 or Envisat must therefore take this relative bias into account if both retracked altimetry heights are used.

Coastal sea level measurements using a single geodetic GPS receiver

In recent years, Global Positioning System (GPS) receivers have been placed on buoys to determine sea surface height. The Naval Oceanographic Office (NAVOCEANO) has become interested in this application for tides work and has sponsored a series of GPS buoy experiments in Mississippi coastal waters. The primary goal of the experiments was the determination of water level with GPS using the WGS84 reference ellipsoid as a tidal benchmark. Also of importance was the determination of the minimum buoy sensor configuration (aside from GPS) required to accomplish this measurement, and to evaluate RTK GPS performance over time and distance. The fully integrated buoy was deployed in 2001, 2002, and 2003. The 2001 RTK results were analyzed for solution availability, solution quality, and solution correlation with other sensor output. Solution availability was high during buoy operational periods; however, the solution quality was hampered by an apparent GPS filter-based anomaly in the receiver RTK processing. Tide gauge comparison indicated subdecimeter-level water level recovery was attainable. Initial processing of 2002 data produced cm-level differences between RTK and tide gauge water levels. These initial results show great promise for the use of RTK buoys in water level recovery, and more data collection and analysis will be undertaken with the 2003 deployment.

Global Positioning System (GPS) spoofing is a common mean to cheat an Unmanned Aerial Vehicle (UAV) which is guided by the INS/GPS integrated navigation system. Detecting the spurious GPS signal is the precondition of anti-spoofing for UAVs. A detection system of GPS spoofing is designed based on dynamics identification. The centroid motion model and attitude motion model were established. The measurement information from inertial measurement units (IMUs) and positioning information from GPS both are visualized responses to the dynamic laws of UAVs. These two types of measurement information were separately used to estimate the dynamics parameters of UAVs online using two Extended Kalman Filters (EKFs). The detection of spurious GPS signals was realized effectively based on the monitoring of relative estimation errors. Numerical simulations demonstrated that the dynamics parameters could be estimated online by the combined filters. And the precision and accuracy of filter #1, which was based on IMU information, were both higher than those of filter #2, which was based on GPS information. When the spurious GPS signal was just zero-order continuous with the true signal at the penetration point, a pulse relative error would occur at the penetration point; when the spurious GPS signal was first-order continuous with the true signal at the penetration point, a step relative error would occur. Furthermore, relative systematic errors matched with the period of spurious signals.

Mobile Geographic Information Systems (GIS) is an outcome of integrating GIS, wireless communication, embeds and mobile computing technologies. Based on .NET Compact Framework and GPRS a Mobile GIS with own patent is developed. Services in the system include: downloading map by General Packet Radio Service (GPRS), defining geographical coordinate system of maps, Global Position System (GPS) connection and information dynamically showing on map, sharing users' GPS information through SMS (Short Message Service) to establishes communication between system users, vector feature map and related property data editing and shortest route planning. The system is now used in field data collection and simple navigation. This paper introduces the implementation of some basic GIS services in mobile environment. And some implemented services are presented and studied in detail.

The region contiguous to the Brazilian Vertical Datum (BVD) lacks of observations and data required for realizing its connection with other vertical networks in the world as required for a Global Vertical Reference System (GVRS). A consistent positional and gravity data base in the region is a fundamental condition for realizing the future Vertical Datum SIRGAS (Sistema de Referencia Geocentrico para as Americas) – DVSIRGAS (Drewes et al., Vertical Reference System, IAG Symposia Series, Springer, Berlin. vol. 124, pp. 297–301, 2002). In the BVD region, placed in the Imbituba harbor, South Brazil, several benchmarks (BM’s) have been lost. In order to improve the distribution of data in this region, we conducted a study on the behavior of the system of three linked lagoons under influence of the ocean dynamics in the region. They cover about 20 × 30 km with a 140 km perimeter in the contiguous region of the BVD. The purpose was to use its mean level as an indicator of a natural equipotential surface close to the geoid (or quasi-geoid). A local geodetic network with about 200 points was established. In this network gravity and precise position with Global Positioning System (GPS) were observed. Some of these points are existing BM’s connected to the BVD. Three tide gauges that recorded the heights of the water level in the lagoons over a period of approximately 3 months were also employed. In this lagoon system, it was possible to determine an approximate equipotential surface from the mean lagoon level (MLL). Then, we determined the shift between the geopotential in the BVD and the lagoon system. Estimations coming from a global geopotential model allowed determination of a provisional value for the Sea Surface Topography (SST) in the BVD.

In this paper we propose a full-view car navigator system which registers full-view spherical images with a digital route map via the GPS (global positioning system) information. A full-view image is captured by a spherical image sensor; the vibration of spherical camera is compensated by the information from an inertial sensor; the rectified spherical image is finally registered to digital route map via GPS information. Since an image sequence is directly registered no 3D model building is required. Since a full-view image is used the urban scene with high buildings can be wholly recorded and there are no limits on view direction. It results in a friendly human interface in our car navigator system.

Defining the source region of the Indian Ocean Tsunami from GPS, altimeters, tide gauges and tsunami models

We analyze data from seven continuous Global Positioning System (GPS) receivers and one tide gauge, all located along the edge of the Greenland ice sheet, to determine vertical uplift rates. We compare our results with predictions based on the ICE‐5G deglaciation model of Peltier (2004). Results from the GPS receiver at Kellyville (−1.2 ± 1.1 mm/a) and from the tide gauge at Nuuk (−2.2 ± 1.3 mm/a), indicate that ICE‐5G overestimates the subsidence rates at those locations by 2.1 and 1.1 mm/a, respectively. Kellyville and Nuuk are located along the southwestern margin of the Greenland ice sheet, and the observed negative uplift rates are consistent with independent evidence that the ice margin along the southwestern edge readvanced during the last ∼8 ka to its current position. The ICE‐5G glaciation‐deglaciation history includes a readvance between the latitudes of 62°N and 72°N. The GPS measurements suggest the ICE‐5G readvance may be too large or mistimed. Our GPS results at Qaqortoq, located at the southern tip of Greenland, suggest a secular subsidence rate of −0.3 ± 1.1 mm/a, while ICE‐5G predicts an uplift rate of 1.0 mm/a. ICE‐5G assumes no ice sheet readvance in south Greenland, including no readvance of the Qassimiut lobe. The difference of 1.3 ± 1.1 mm/a can tentatively be explained as due to a ∼33 km readvance of the Qassimiut lobe during the last ∼3 ka. For the other GPS sites, the observed/predicted uplift rates are 3.6 ± 1.1/−0.1 mm/a at Thule, 0.0 ± 1.1/2.0 mm/a at Scoresbysund, and −0.4 ± 1.1/−1.7 mm/a at Kulusuk. For Thule, Kulusuk, and Scoresbysund the differences between the observed and predicted rates are on the order of 1.3–3.7 mm/a, though with opposite signs, and indicate that ICE‐5G does not exactly reproduce the correct rebound signal at those locations.

In this paper, we develop an alternative navigation system for Unmanned Aerial Vehicle (UAV) in Global Positioning Systems (GPS)-denied environment. We use two image inputs, one is acquired with an on-board camera placed on the UAV (which is the large-area image) and the other is from satellite images (which is small known image) with GPS information. We use a convolutional neural network (CNN) architecture based on Oxford’s Visual Geometry Group network (VGG-16) and utilize normalized variant mutual information between these two images to obtain position of the UAV. Satellite images are labelled and given to the UAV. When GPS information is lost, our algorithm starts to function and images from UAV camera are searched whether satellite image is seen by cameras on UAV image or not. If the UAV is in that area, our algorithm finds the GPS information from satellite image data.

The relationship between heat content and sea surface height (SSH) in the global ocean on the interannual time scale is examined with satellite altimeter measurements, historical hydrography, and model assimilation outputs. Results show that correlation between altimetric SSH and heat content in the upper 700 m calculated from Ishii data is geographically nonuniform. In the tropical ocean, heat content and SSH are strongly correlated and exhibit nearly the same interannual variations. In the polar ocean, their correlation is relatively weak. Further analysis with Simple Ocean Data Assimilation outputs shows that such nonuniform distribution is not from dynamical origin but from the limited integral depth selected to calculate heat content. The integral depth of 700 m is inadequate to capture variation of the deep main thermocline in the polar region. The halosteric effect also contributes to the nonuniform pattern of correlation, because saline contraction becomes significant in the polar ocean owing to low temperature.

Sea level variations (SLV) can be measured by tide gauges (TG) at the coast and by altimeters onboard satellites. The former measures the SLV relative to the coast, whereas altimetry provides the SLV with respect to a geocentric reference frame. The differences between SLV measurements from these two techniques can be used as an indirect assessment of vertical crustal motions at the TG sites. In this study, we exploit this idea, analyzing differences between sea level signals as measured by altimetric missions (TOPEX/Poseidon and Jason-1) and by 47 TG stations along northern Mediterranean coasts for the period 1993–2007. This allows us to estimate the vertical land motion along these coasts at the TG sites in this time window. For those sites where the TG is co-located or has a nearby global positioning system (GPS) station, these estimates are compared with the vertical rates derived from GPS measurements. Our results on vertical ground motion along the Mediterranean coast provide a useful source of data for studying, contrasting, and constraining tectonic models for the region.

<p>The absolute and relative accuracy of sea surface heights derived from six altimeter missions (Jason-1/2/3, Envisat, Saral, Sentinel-3A) is evaluated at five GNSS-controlled tide gauge stations in the German Bight (SE North Sea). The precision of the total water level envelope (TWLE) is assessed for the period 2000 to 2019 based on RMS errors and explained variances. The comparison is based on TWLE instead of dealiased sea level data since the tidal and barotropic dynamic is not known with sufficient accuracy in this area. The tide gauges are partly located at the open sea, partly at the coast close to mudflats. The tide gauge data is available every minute, the 20 Hz level 2 altimetry data is interpolated to virtual stations at distances between 2 and 15 km to the tide gauges. The altimeter data is based on standard retrackers, the correction models are adjusted to coastal applications and exclude the corrections for ocean tides and dynamic atmosphere to allow a direct comparison to the tide gauge data. To account for slight differences of the tidal dynamics between gauge and altimetry an optimal time shift and scale between each pair of locations is estimated and applied. This tidal correction improves the RMS errors by 15-75%. The explained variances are excellent at all stations (> 96%). The resultant RMS errors are mainly between 2-5 cm depending on location and mission. The RMS errors rise up to 10 cm where coastal dynamics play a dominant role or the altimeter approaches the land very closely (<7 km). The accuracy of the absolute biases is strongly dependent on the knowledge of the mean sea surface heights in the region.</p>